A. Field of the Invention
The present invention relates to antennas to transmit and receive ultra-high frequency radio signals. More particularly, the invention relates to novel transportable X-WING type UHF SATCOM antennas which are attachable to a vehicle, ship or other support structure, and which are highly resistant to impact damage and readily repairable in the field.
B. Description of Background Art
Government agencies such as U.S. military services that utilize personnel operating in remote field locations have a need for instantaneous, reliable communication systems. Such systems are required for conveying data between personnel in field locations and fixed command and control sites. As a practical matter, communication systems which meet the various requirements for reliable communications of the type alluded to above generally utilize radio transceivers. Thus, the U.S. military services and other governmental agencies typically use for their communications between remote field location, and between remote field locations and command and control sites, small, readily transportable radio transceivers. Such transceivers, which are typically installed in vehicles or ships, usually operate at power levels of 200 watts or less. To achieve long distance communication capability, and to avoid line-of-sight signal transmission obstructions such as mountainous terrain, portable communication transceivers used for applications such as those described above often utilize a transponder located in an earth-orbiting satellite, and are hence used in communication systems referred to as Satellite Communication (SATCOM) systems.
Radio transceivers of the type described above must of course use an antenna to transmit and receive radio signals through space. Thus, transportable transceivers which are used to communicate over long distances and/or rugged terrain where line-of-sight communication is not feasible often utilize transmissions between an earth-orbiting satellite to provide the needed range and terrain obstruction avoidance. For such applications, small SATCOM antennas mountable to vehicles, ships or portable shelters and operable in ultrahigh frequency (UHF) radio bands are frequently used.
Vehicle mountable SATCOM antennas currently in use are required to have a reasonably high gain in UHF radio bands located generally between about 225 MHZ and 400 MHZ. Typical SATCOM antennas are constructed to utilize circularly polarized signals. Circular polarization is required for satellite communication because ionized particles in the upper part of the atmosphere known as the ionosphere rotate the plane of polarization of a linearly polarized radio signal, thus causing a polarization mismatch in linearly polarized antennas. One type of SATCOM antenna in common use has a “turnstile” type external appearance, or “form factor,” which includes a central straight, longitudinally disposed mast that has protruding radially outwards from the upper end of the mast four radiating elements which are spaced circumferentially apart at 90-degree intervals. The active part of each radiating element which is effective in transmitting or receiving radio frequency electromagnetic waves is an elongated straight electrical conductor, which may be in the form of a blade or rod. The conductors of one pair of diametrically opposed elements comprise an electric dipole antenna that is electrically connected to a first port of a hybrid antenna coupler network. The conductors of a second pair of elements oriented at 90-degrees to the first pair comprise a second electric dipole antenna, and are connected to a second port of the antenna coupler network, which is shifted in phase 90-degrees from the first port by circuitry in the coupler network. This arrangement results in the transmission of a circularly polarized signal. The arrangement also enables the conductors of the elements of the antenna to intercept and receive at relatively high gain radio signals of various polarizations, when the antenna is operated in a receive mode, with no transmitting signals applied to the radiating elements.
When viewed from above or below, the radiating elements of SATCOM antennas of the type described above, which typically consist of four conductive rods which extend perpendicularly outwards from the antenna mast, form an X-shaped pattern. Thus such antennas are commonly referred to as “X-WING” antennas.
Portable X-WING SATCOM antennas which are intended for use in field operations are typically mounted to the hood, roof or the fender of a vehicle, such as a HumVee. Therefore, the outwardly protruding radiating elements of such antennas are subject to impact damage from contact with low-hanging tree branches, for example. Such damage can degrade or completely destroy the functionality of X-WING antennas currently in use, and thus jeopardize the success of missions which require reliable communications implemented with the antenna. Accordingly, it would be desirable to provide an improved X-WING UHF SATCOM antenna which had superior impact resistance. For the same reason, it would be desirable to provide an improved impact resistant X-WING UHF SATCOM antenna which utilized field-replaceable radiating elements. Also, it would be desirable to provide an improved UHF SATCOM antenna which utilized radiating elements that could be readily replaced in the field if damaged.
An additional problem with prior art X-WING UHF SATCOM antennas is the large amount of container space which is required to store and ship such antennas to the field. As can be readily envisioned, the form factor of an X-WING antenna, which includes a longitudinally elongated cylindrical mast that has four straight rod-like radiating elements approximately equal in length to the length of the mast protruding radially outwards from the upper end of the mast, requires essentially a storage or shipping space having the shape of a rectangular block whose height is equal to the mast height, and whose base sides are equal to the length of the radiating elements. Accordingly, it would be desirable to provide an improved X-WING UHF SATCOM antenna which could be configured to a smaller space for shipping, and quickly and easily be re-configured to an operational configuration in the field.
The foregoing limitations of prior art X-WING UHF SATCOM antennas, and the foregoing improvement objectives, were motivating factors for the present invention.